Which career path is the most meritocratic (eat what you kill) and ability to reach to the top at a young age?

The problem with most corporate jobs, many areas of finance being no exception, is that it's very hard to directly show your "value", so a lot of performance is based on perception rather than tangible value added. The closest things you'll get to directly showing your value within finance are:

1. Sales-based jobs: Any sales role where you can directly show how much revenue you generated and can demand a portion of that if you're valuable enough. I'm less familiar with the S&T side of the house, but another finance adjacent area to this would be financial ERP sales to CFOs/Controllers

2. Owning a P&L: similar to sales, but likely requires you to be a junior executive. That said, if you can somehow attribute your efforts to directly increasing the company's bottom-line, you'll be better able to name your price or at least get a nice bonus for your rewards. This is a lot more nebulous than the other options still because an initiative's success will also be dependent on whether or not departments outside of finance can execute the plan you lay out. 

3. Asset Mangement: If you can prove you know how to best allocate peoples' money, you can get paid a handsome fee for the privilege of managing their money and share in some of the outsized gains you produce as well. The examples for this path are obvious (trading, HF, etc.)

Outside of that, I'd say that the corporate world isn't super meritocratic IME. The job in Corporate Finance (my area of play) at least is so dumbed down that just about anyone with a modicum of work ethic can do the work. I sort of wish I had the personality to do something like sales since it scales so much better for the reasons I mentioned, but I'm just not built that way.